"Complementarity" is the diplomatic and legal framing used by supporters of the Treaty on the Prohibition of Nuclear Weapons (TPNW) to argue that it strengthens the existing nuclear order built around the Treaty on the Non-Proliferation of Nuclear Weapons (NPT). The claim is that the TPNW operationalises Article VI of the NPT, under which parties pledged to pursue negotiations in good faith on effective measures relating to nuclear disarmament.
Proponents — including Austria, Ireland, Mexico, New Zealand, South Africa, and the International Campaign to Abolish Nuclear Weapons (ICAN) — argue that the TPNW fills a long-standing "legal gap" by categorically prohibiting development, possession, stationing, and use of nuclear weapons, while leaving NPT safeguards, non-proliferation norms, and the right to peaceful nuclear energy untouched. The TPNW's preamble explicitly reaffirms the NPT as the "cornerstone" of the nuclear disarmament and non-proliferation regime, and Article 3 requires non-nuclear-weapon states parties to maintain at minimum their existing IAEA safeguards obligations.
Critics — chiefly the NATO nuclear-sharing states, the five NPT nuclear-weapon states (P5), and umbrella allies such as Japan, Australia, and South Korea — counter that the TPNW risks creating competing legal standards, weakens consensus diplomacy at NPT Review Conferences, and ignores the security environment that sustains deterrence. A joint statement by the US, UK, and France in 2017 rejected the TPNW outright.
The complementarity debate became central at the 2022 NPT Review Conference (which ended without a consensus outcome) and at the First Meeting of States Parties to the TPNW in Vienna (June 2022), whose declaration and action plan repeatedly framed the two treaties as mutually reinforcing. The issue resurfaces at every NPT PrepCom and is a recurring flashpoint in First Committee debates of the UN General Assembly.
Key points often raised in MUN and policy briefs:
- Legal: Does the TPNW add obligations or merely restate them?
- Political: Does parallel treaty activity dilute pressure on the P5?
- Practical: Can safeguards regimes (CSA, Additional Protocol) coexist coherently?
Example
At the 2022 NPT Review Conference in New York, Austria and Ireland argued that the newly entered-into-force TPNW was complementary to the NPT, while the United States and France maintained it risked fragmenting the non-proliferation regime.
Frequently asked questions
Yes. The TPNW is designed for non-nuclear-weapon states already party to the NPT, and most of its signatories are also NPT parties. The two treaties impose overlapping but not contradictory obligations on them.
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